Many reports of even factory produced poly lowers having issues. Millions of them also out there working fine as well, so not sure that really helps, but just somethign to keep in mind...
If I was going to take the time and energy to do a 80% I would Def do it in aluminum, just more durable, less concern about caliber or treatment/abuse. Also make sure to buy one already coated, otherwise you are just looking at another chunk of change to get it coated after you mill it.. I think typically $50-75 just to coat a bare lower.